The U-937 monocyte-like cultured cell, a model for circulating monocytes, binds and internalizes insulin in an analogous fashion to target cells such as liver cells. The U-937 monocyte internalizes insulin at a high rate and the addition of monensin to insulin markedly augments down regulation of receptors. By contrast, the IM-9 lymphocyte which internalizes insulin poorly, shows essentially no additional effect when monensin is added. The drug colchicine which inhibits endosomal transfer to lysosomes in hepatocytes has been studied. There is no localization to an additional compartment such as the Golgi with respect to processing of an internalized ligand. The internal route appears to go from the endosome of one or more types to the lysosome of one or more types; the recycling process would appear to occur from either of the two membrane-bounded structures.